1. Technical Field
The present disclosure relates to a gonio-spectroradiometer for measuring a total spectral radiant flux (unit: W/nm), which is one of the optical properties of a light-emitting device, at high speed.
2. Description of the Related Art
Total spectral radiant flux (unit: W/nm) of a light source is the sum of radiant flux (unit: W) per unit wavelength emitted in all direction. The total spectral radiant flux provides the information on total optical power at each wavelength of a light source. Accordingly, most of the optical properties of light sources such as luminous efficacy (unit: lm/W), total luminous flux (unit: lm), chromaticity, color rendering index, and a correlated color temperature (unit: K) can be obtained from the total spectral radiant flux. Accordingly, the total spectral radiant flux is important in performance evaluation of lighting apparatuses. In particular, it is more important to measure the total spectral radiant flux with respect to ultraviolet and infrared light sources.
In general, methods for measuring the total spectral radiant flux may be classified into a method for measuring and summing spectral irradiances (unit: W/m2/nm) or spectral radiant intensities (unit: W/sr/nm) at respective positions around a measurement target light source and a method for comparing a total spectral radiant flux standard lamp of a known total spectral radiant flux value with a measurement target lamp using an integrating sphere.
The former method is a method using a gonio-spectroradiometer. Spectral radiant intensity or spectral irradiance must be measured and summed while a spectroradiometer of which the spectral radiant intensity or the spectral irradiance scale is calibrated rotates around a light source. In this case, a mechanical apparatus is required to rotate the spectroradiometer around the light source. Since it takes considerable time for the mechanical apparatus to rotate the spectroradiometer, measurement speed (measurement time) is very low (at least a few hours). However incidentally, an angular distribution of luminous intensity of the light source may be measured and accurate measurement for light sources having non-isotropic angular distribution of luminous intensity may be performed. In addition, it is possible to avoid an influence of fluorescence that often occurs when an ultraviolet light source is measured using an integrating sphere. Therefore, it is very efficient for an ultraviolet light source. In general, a gonio-spectroradiometer is used as a primary standard for measuring a total spectral radiant flux.
The latter method is a method using an integrating sphere spectroradiometer. Unlike the gonio-spectroradiometer, the integrating sphere spectroradiometer requires a standard lamp whose total spectral radiant flux is known. Since the measurement is basically a comparison measurement, measurement speed (measurement time) is very high (less than an few minutes). However, an additional uncertainty component related to angular distribution of luminous intensity gives an effect. Accordingly, when angular distribution of luminous intensity of a light source is significantly different from a point-like, isotropic light source, accuracy of an integrating sphere spectroradiometer is lower than that of a gonio-spectroradiometer. In addition, fluorescence on inner surface of the integrating sphere makes it very difficult to apply the latter method to an ultraviolet light source of 400 nm or less. Moreover, since a total spectral radiant flux standard lamp required to calibrate integrating sphere spectroradiometer can be calibrated only by the gonio-spectroradiometer, the integrating sphere spectroradiometer is used as a secondary standard.